Rock bits used for drilling oil wells and the like commonly have a steel body that is connected at the bottom of a drill string. Steel cutter cones are mounted on the body for rotation and engagement with the bottom of a hole being drilled to crush, gouge, and scrape rock for drilling the well. One important type of rock bit, referred to as a “milled tooth” bit, has roughly trapezoidal teeth protruding from the surface of the cone for engaging the rock.
Conventional milled teeth are made from steel, and are “hardfaced” for the purpose of providing an improved level of wear protection. Such milled teeth can be completely hardfaced, or be selectively hardfaced to provide a desired self-sharpening effect during drill bit operation. While conventional completely hardfaced teeth are known to offer an adequate level of protection to the underlying steel tooth during the drilling operation, the placement of hardfacing over the entire tooth increases the effective area of the tooth is theorized to have a limiting effect on the ROP.
Conventional self-sharpening teeth are specifically designed having hardfacing disposed along strategic surface areas of the teeth to produce a preferential wearing of the nonhardfaced surfaces. While this combination of wear protected and preferential wearing surfaces produces a sharpened structure known to improve ROP, it is known that some of the nonhardfaced surfaces can leave the teeth vulnerable to erosion cracking, which can eventually cause the teeth to break. Such breakage can have a detrimental impact on achieving the desired ROP.
The term “hardfaced” is understood in industry to refer to the process of applying a carbide-containing steel material (i.e., conventional hardmetal) to the underlying steel substrate by welding process, as is better described below. Thus, the terms “hardfaced layer” or “hardfacing” are understood as referring to the layer of conventional hardmetal that is welded onto the underlying steel substrate.
Conventional hardmetal materials used to provide wear resistance to the underlying steel substrate usually comprise pellets or particles of cemented tungsten carbide (WC—Co) and/or cast carbide particles that are embedded or suspended within a steel matrix. The carbide materials are used to impart properties of wear resistance and fracture resistance to the steel matrix. Conventional hardmetal materials useful for forming a hardfaced layer on bits may also include one or more alloys to provide one or more certain desired physical properties. As mentioned above, the hardfaced layer is bonded or applied to the underlying steel teeth by a welding process.
The hardfaced layer is conventionally applied onto the milled teeth by oxyacetylene or atomic hydrogen welding. The hardfacing process makes use of a welding “rod” or stick that is formed of a tube of mild steel sheet enclosing a filler which is made up of primarily carbide particles. The filler may also include deoxidizer for the steel, flux and a resin binder. The relatively wear resistant filler material is typically applied to the underlying steel tooth surface, and the underlying tooth surface is thus hardfaced, by melting an end of the rod on the face of the tooth. The steel tube melts to weld to the steel tooth and provide the matrix for the carbide particles in the tube. The deoxidizer alloys with the mild steel of the tube.
While this hardfacing process is effective for providing a good bond between the steel substrate and the conventional hardmetal material, it is a relatively crude process that is known to adversely impact the performance properties of the hardfaced layer. The hardfacing welding process itself generates certain welding byproducts that can and does enter the applied material to produce an inconsistent material microstructure. For example, the welding process is known to introduce oxide inclusions and eta-phases into the applied material, which function to disrupt the desired material microstructure. Such disruptions or inconsistencies in the material microstructure are known to cause premature chipping, flaking, fracturing, and ultimately failure of the hardfaced layer. Additionally, the welding process and associated thermal impact of the same can cause cracks to develop in the material microstructure, which can also cause premature chipping, flaking, fracturing, and ultimately failure of the hardfaced layer.
Additionally, the hardfacing process of welding the carbide-containing steel material onto the underlying substrate makes it difficult to provide a hardfaced layer having a consistent coating thickness, which ultimately governs the rate of wear loss for the steel material, and the related service life of bit.
Example conventional hardmetal materials, useful for forming a conventional hardfaced layer, typically comprise in the range of from about 30 to 40 percent by weight steel, and include carbide pellets and/or particles having a particle size in the range from about 200 to 1,000 micrometers. Examples of conventional materials used for forming hardfaced layers can be found in U.S. Pat. Nos. 4,944,774; 5,663,512; and 5,921,330. The combination of relatively high steel content and large carbide particle size for such conventional hardmetal materials dictate that the mean spacing between the carbide pellets within the steel matrix be relatively large, e.g., greater than about 10 micrometers. It is believed that this relatively large mean spacing of carbide particles within the conventional hardmetal material causes preferential wear of the steel matrix that is known to lead to uprooting and removal of the carbide particles. Such wear loss is known to occur along the milled tooth tip at high stress locations during drilling and functions to accelerate loss of the hardfacing, which is a predominant failure mechanism for hardfaced bit surfaces, thereby limiting the service life of such bits.
It is, therefore, desirable that a milled tooth be constructed in a manner providing a desired degree of wear resistance against erosion, while at the same time providing improved ROP when compared to conventional completely hardfaced milled teeth and conventional self-sharpening milled teeth. It is desired that such milled tooth be capable of providing a self-sharpening feature. It is desired that the milled teeth be constructed having a wear and fracture resistant material alternative to conventional hardfacing that avoids the undesired effects of hardfacing, e.g., that avoids the undesired impact on the material microstructure due to the thermal effect and introduction of unwanted byproducts inherent in the welding process, that can adversely impact drill bit surface performance properties. It is desired that such alternative wear and fracture resistant material be designed and/or applied onto the surface of a rock bit in such a manner as to provide improved properties of dimensional consistency and accuracy, e.g., a substantially consistent wear resistant surface thickness, when compared to conventional hardfaced materials. It is also desired that such wear and fracture resistant material be engineered in such a manner as to avoid the problem of preferential wear loss that is inherent to conventional hardmetal materials.